BQ24650: Output capacitor selection

Hi Richard, 

   It is not the specific output capacitance that affect the feedback compensation loop, but the resonant frequency of the LC output filter (section 8.3.8 of datasheet). This link in the software section of the product page has a calculator tool that can help you in deciding the right value: http://www.ti.com/product/BQ24650/toolssoftware

Hi Kedar.

The spreadsheet recommends a 15uH inductor and 10uF cap for a 1A charge current.

I understand that it's the combination of L and C that is important. But the inductance value is constrained by other factors, namely duty cycle and charge current, which, in turn, constrains the capacitance value in light of the resonant frequency requirement. My problem is that the boost supply needs a larger input capacitor, and the charger buck supply can't accommodate that larger value. So, my question is still the same. What happens if the resonant frequency of the output circuit is lower than 10KHz? And, is it sufficient to isolate that output using a low-value resistance with the boost input cap on the other side?

Hi Kedar.

The spreadsheet recommends a 15uH inductor and 10uF cap for a 1A charge current.

I understand that it's the combination of L and C that is important. But the inductance value is constrained by other factors, namely duty cycle and charge current, which, in turn, constrains the capacitance value in light of the resonant frequency requirement. My problem is that the boost supply needs a larger input capacitor, and the charger buck supply can't accommodate that larger value. So, my question is still the same. What happens if the resonant frequency of the output circuit is lower than 10KHz? And, is it sufficient to isolate that output using a low-value resistance with the boost input cap on the other side?

Hi Richard,

     You are correct that your input capacitance of the boost converter would add to the output LC resonant frequency of BQ24650. It is not recommended to operate charger outside of recommended frequency range as the internal feedback loop has inbuilt poles and zeroes to guarantee stability when the double pole of the LC is centered between 12-17 kHz. Operation of the buck charger has not been validated outside of this range. 

   What is the input capacitance you have to add for your boost converter? Lowering the inductance can also help keep LC resonant frequency in recommended range. Choosing inductance is a tradeoff, as lower inductance has lower DCR (less effect on efficiency) but higher inductor ripple current and faster response to transients. 15uH is recommended for 1A charging as voltage drop across higher DCR at lower currents can be accepted to get lower ripple current and faster transient response, but at higher currents the effect on efficiency is increased.

   Also yes, using a low value resistance between the output capacitors of BQ24650 and input capacitors of boost converter, can decrease the effect of the boost converter's input capacitors on the stability of BQ24650 but has not been quantified or validated.